ByKen Thomas, Associated PressJanuary 28, 2016

Fairfield, Iowa — Democratic presidential candidate Bernie Sanders launched a tough new television ad critical of Goldman Sachs' role in the financial meltdown and payment of speaking fees as he ramped up his criticism of Hillary Clinton's ties to Wall Street, seeking an upset victory in Monday's first-in-the-nation Iowa caucuses.

The 30-second ad, which circulated Thursday ahead of its scheduled Friday release, makes no mention of Ms. Clinton, who received more than $600,000 in speaking fees from the Wall Street firm. But it comes amid a push by Mr. Sanders to make policing the financial sector central to his presidential campaign.

"How does Wall Street get away from it? Millions in campaign contributions and speaking fees," the ad says, describing Goldman Sachs' recent $5 billion settlement from its role in the financial crisis. "Our economy works for Wall Street because it's rigged by Wall Street and that's the problem."

Sanders only appears at the end of the ad to approve the message. But he has driven a similar argument in recent weeks, seeking to capitalize on concerns by Democrats that Wall Street has become too influential in the nation's economy and political system.

The so-called "democratic socialist" vows to break up the country's biggest financial firms within a year if he's elected president and limit banking fees placed on consumers. During the last presidential debate, Sanders pointed to Clinton's Goldman Sachs speaking fees and took a swipe at Clinton during a Wednesday night event in Mason City, noting that Clinton was raising money at a Philadelphia investment firm while he was campaigning with Iowans.

Clinton argues that her financial regulation proposals would do more to crack down on industry abuses and rein in risky behavior within the shadow banking sector.

"We've got some work to do and I want to do it by getting the wealthy to pay their fair share for the benefits they have enjoyed by being Americans," Clinton told several hundred voters gathered at a middle school in Newton, Iowa.

Responding to the ad, Clinton's campaign said Sanders was shifting into attack mode and breaking his pledge not to run negative advertising.

"This last-minute sneak attack from the Sanders campaign is clearly meant to plaster the Iowa airwaves in the days before the caucus with negative ads slamming Hillary Clinton, without giving our campaign time to respond," said Matt Paul, Clinton's Iowa director. "It's a cynical political ploy."

The campaigns have had feisty exchanges as Iowa's leadoff contest nears. A new Clinton ad says she would build upon President Barack Obama's health care law, "not start over," and defend Planned Parenthood, "not attack it," messages that reinforce charges she has made against Sanders. In a statement, Sanders said the ad "completely distorts my record."

Sanders is trying to upset Clinton in Iowa, where her advantage has narrowed in recent weeks, and then claim victory in New Hampshire, where polls show him with a steady lead in the Feb. 9 primary. Traveling by bus through Iowa, the senator predicted a win in Iowa if the turnout was large. "If the turnout is not large, we're going to be struggling," he told staffers in Des Moines.

It came on a day when Sanders' campaign conducted damage control in Nevada and released the senator's medical records, something he had vowed to do before the Iowa caucus.

Leaders of the powerful Culinary Workers Union in Las Vegas – which remains neutral in the race – condemned Sanders' campaign staffers who falsely claimed an affiliation with the union as a way to gain access to its members at casino employee dining rooms. Sanders campaign manager Jeff Weaver said the campaign reminded staffers it was inappropriate "and that they should not do it again."

Earlier, Sanders' campaign distributed a one-page letter from the senator's doctor, who declared the candidate to be in "overall very good health."

It said Sanders had been treated in the past for ailments like gout, high cholesterol, and laryngitis. He underwent hernia surgery earlier this year but is in good health, his doctor said.

"You are in overall very good health and active in your professional work, and recreational lifestyle without limitation," wrote Sanders' longtime doctor, Brian Monahan, the attending physician of the US Congress.

If elected, Sanders would be the oldest president to assume office, surpassing Ronald Reagan who was 69 when he entered the White House. Sanders is about six years older than Clinton, who released her medical records in July. Her doctor said at the time she was in "excellent physical condition."